1. Field of the Invention
The present invention pertains generally to correcting for viewing aberrations produced by electromagnetic radiation passing through a surface, and, more particularly, to correcting for refraction of radiation that has passed through an axisymmetric surface.
2. Description of Related Art
There are many situations in which electromagnetic energy (e.g., infrared light) must pass through a surface (e.g., window) before actuating other apparatus. In the control of a missile, for example, infrared energy from an external target on being sensed at the missile serves as a means for identifying and locating the target to enable guiding the missile to the target. Where the windows used have a substantial curvature to them (so-called domes), this tends to produce a prohibitively large refraction of the electromagnetic energy when the angle of radiation incidence varies significantly from 90 degrees, giving the impression to the control apparatus that the source of the energy is located at a different position than where it is in actuality.
A known technique for solving this problem at the present time is to construct the window of either a flat plate or a plate having a spherical surface. In addition, there are other approaches in which a fixed correction is built into an axisymmetric window according to an assumed direction that target information can be expected to take which, of course, means that a variable amount of error will have to be tolerated, dependent upon how different the actual target direction is from the assumed direction. The use of flat or spherical surfaces, however, produces an undesirably high aerodynamic drag if they are allowed to be exposed directly to the environment, so that if used will require a removable shroud which is positioned over the window until quite late in the missile trajectory when it is jettisoned. In the latter case, the shroud precludes use of, say, an infrared sensor at long range, and the jettison mechanism for removing the shroud involves additional weight, is awkward and restricts operation somewhat.